Particulate materials such as non-caking coals, green petroleum coke, anthracite coal, bituminous coal, wood products and other carbonaceous materials, dolomite, limestone and the like must often be de-volatilized and calcined for further use. Rotary hearth furnaces are commonly used for that purpose. The raw material is charged upon the hearth at its circumference and rabbles fixed in the stationary roof cause the material to move toward the center of the hearth, through which it is discharged as the hearth rotates. Heat is generated in the furnace chamber sufficient to raise the material to a calcining furnace temperature which in the case of petroleum coke the like materials is between about 1250 degrees C. and 1500 degrees C. As the coke or other material travels toward the center of the hearth, it rapidly heats up and its volatile constituents are driven off. The material so calcined however is physically weak and fragments easily. For some purposes that characteristic is undesirable. In the manufacture of electric furnace electrodes from petroleum coke for example it has been found that the easily fragmented coke produces electrodes relatively low in mechanical strength.